The need to provide environmentally correct and cost effective solutions for the refuse generated in the Unites States became apparent in the late 1960's. At that time, refuse disposal was mainly by land filling and to a lesser extent incineration. It was recognized that landfill space was a finite resource and that refuse itself was an inherent fuel resource that could be utilized to displace other more costly fuel sources in the generation of process steam and electricity.
A refuse to energy plaint is composed of several separate and distinct areas. These include: (1) refuse receiving, handling and storage; (2) refuse combustion; (3) heat recovery and electrical generation; and (4) environmental control. The refuse combustion system is of particular interest herein and it typically is composed of the following: (1) refuse feed hopper and chute, (2) ram feeder, (3) grate combustion system, (4) lower furnace combustion zone, and (5) grate ash discharge system. Current conditions require that these systems operate over extended periods of time with limited maintenance. Initial operating results for these types of facilities exhibited extensive maintenance and downtime.
Refuse is introduced to the grate combustion system via a charging hopper and feed chute. Typically, a crane, or in some cases, a front end loader picks up a quantity of refuse from the site receiving and storage area and deposits it into the charging hopper. This charging hopper has a large plan area to facilitate this operation and acts as a funnel to feed the refuse to the feed chute. The feed chute typically is rectangular in cross section and has slightly divergent sides. The width of the feed chute approximates the width of the grate to facilitate uniform refuse fuel flow across the unit. The feed chute and the lower part of the charging hopper are always kept full of refuse to maintain a seal between the combustion zone within the furnace enclosure and ambient. i.e. the exterior of the furnace enclosure.
Refuse from the feed chute exits to a flat, table top surface, directly below. This surface provides a staging area for the refuse to move out onto the grate in a controlled manner beginning the combustion process. A ram feeder, which is a plow-type device, operates on top of this table and is hydraulically driven at a predetermined speed to push the refuse onto the grate. The feeder is considered a volumetric flow controller as it pushes a volume of refuse equal to its height by the plan area of the feed chute discharge opening. The feeding portion of the unit involves the group operation of several parallel ram feeders across the full width of the grate system to insure equal fuel loadings across the unit. Accordingly, refuse is pushed off the table and onto the grate system to start the combustion process within the furnace enclosure. Drying and ignition of the refuse occurs a short distance downstream on the grate system.
The grate system in a refuse fired furnace provides the necessary support of the refuse fuel bed while being transported through the combustion sequence. In order for satisfactory combustion to occur, air must be supplied from below the fuel bed and be equally distributed across the width of the grate system and depth of the fuel bed in the unit. This combustion air also provides the cooling of the grate system necessary to maintain its integrity.
Typically, the grate system is a forward moving reciprocating-type with the grate blocks each inclined at an angle of 18 degrees from the horizontal and with alternating transverse rows (relative to the direction of refuse flow) of stationary and reciprocating grate blocks. Each row of grate blocks overlaps the row ahead of it to provide the grate system surface. The alternate rows which move in a reciprocating manner are supported by a moving structure providing the reciprocating motion. Attachment to either a fixed or moving guide is made at the back part of the grate blocks which is located below the prior overlapping block.
The grates are arranged on modular parallel lanes in the direction of fuel flow and are constructed in several modules across the unit width. Individual modules typically are considered as being arranged in zones relating to the combustion sequence, numbered in the direction of fuel flow, and each has its independent air supply. This accommodates the varying combustion sequences which include drying and ignition, main combustion and burnout which occur as the fuel travels from zone to zone. Each zone is usually two or three modules across the furnace width by five modules in the direction of fuel flow down the grate. Each module is usually made up of eight rows of grate blocks with each row having 24 individual grate blocks. Each module also has a press plate located at and defining each of its lateral sides. Furnace units operating with such prior art grate blocks have experienced excessive grate system maintenance and a degradation in combustion performance over a narrow time frame.
In grate modules using a prior art grate block, air distribution below the fuel bed is typically provided by two 1/2" round holes located in the leading face of each individual grate block. The air would be provided at a positive pressure from a fan to a plenum below the grates, and actual distribution to the fuel bed would be through these holes in the grate blocks directly to the refuse being transported and combusted. Actual experience defined a consistent pattern that after a 12-14 week operational period, approximately 90% of the air holes were plugged causing improper combustion and a lack of cooling of the grates. The plugging would be most severe in the center area, across the width of the grate system, where combustion demands are the most severe due to inherent heavier fuel bed loadings. Resultant actual air distribution would be excessive along the side walls with short circuiting causing clinkering and limiting the ability of the furnace unit to remain in service.
Another important consideration in achieving even air distribution across the grate system is proper sealing between grate modules and along the side walls of the furnace unit adjacent to the grate modules. Without proper scaling, channeling occurs with the associated problems noted above relative plugging of the air hole in the grate blocks.
In this regard, it is first noted that each grate module operates as a controlled and independent unit relative to the adjacent modules. Maintaining the alignment of each row of grate blocks within a module requires that the grate blocks fit tightly side by side. The tight fit also contributes to proper air distribution. The tight fit is usually accomplished by a pair of tension rods located below each row of grate blocks. In reciprocating rows, each rod is attached at one end to one of the end grate blocks in such rows (e.g. the 1st or 24th block). In stationary rows, each rod is respectively attached to a corresponding one of the two press plates which define the lateral sides (relative to refuse flow) of each module. This rod is essentially a rod with a turnbuckle at the other end thereof located about midway in each row to allow tightening to a pre-determined torque to provide the desired tension during operating conditions. However, the tension rod is therefore manually fixed to the pre-determined tension while the unit is in the cold condition. During operation, the rod is cooled by combustion air flowing beneath the grate, while the grate is subjected to temperatures relative to the fire above. Accordingly, the tension rods exhibited great sensitivity to the various conditions including original manual tensioning errors and differential temperature and expansion conditions occurring during operation and shutdown of the furnace and the transition therebetween. Failure of this tension rod would cause the entire row of grate blocks to lift and require a shutdown to repair.
A design called the sidewall grate roof elements and grate sidewall seals provide for the sealing between the modules adjacent the furnace sidewalls and the respective furnace sidewall. The purpose of this roof element and particularly the grate sidewall seal is to provide a seal for combustion air which flows through the grate system. The roof element and grate sidewall seals located at the junction of the grate system and the lower furnace sidewall must accommodate both grate system expansion as well as furnace expansion. The grate sidewall seal itself initially remains in a fixed position relative to the associated press plate. However, during operation, the lower furnace moves due to expansion approximately 1/2" in a horizontal direction away from the seal and roof element creating a gap between the roof element and the side wall. The prior art design allowed a foreign material (e.g. ash and refuse) to lodge in this area when the unit was operating (in a hot condition) and subsequent failures of this seal would occur when cycling the unit between operating and shutdown conditions. The lodged foreign material impeded the return of the seal to its original position during the shutdown condition. Eventually, the lodged foreign material would "push" the seal out of position breaking the seal, i.e. a sealing condition could not be maintained, causing channeling and associated air distribution problems.
Similarly, grate roof elements are installed between the press plates of adjacent grate modules. However, different types of sepals are used which are rope-like in configuration and are inserted into grooves in the outward face of the press plates. The seals in this position seal against the vertical sides of the grate roof elements. Accordingly, the seal problem noted above is not present between modules. However, the grate roof elements, including the sidewall grate roof elements, are attached to the respective module supporting structure by a yoke and pin arrangement. If for any reason maintenance was necessary on these elements, e.g. replacement due to wear or breakage, the grate modules adjacent to the affected grate roof elements would have to be disassembled and the grate blocks adjacent thereto removed to gain access to the yoke and pin assembly to effect replacement thereof.
Therefore, a need exists to alleviate or eliminate the foregoing problems associated with the design of component parts of the grate combustion system.